This invention relates to a noise generator used for a noise environmental test of a mobile communication machine and in particular to a noise generator preferred for a noise environment test of a mobile communication machine compatible with CDMA (Code Division Multiple Access).
In recent years, the number of users carrying mobile communication machines such as portable telephones and pager units has increased rapidly with the development of mobile communication technologies. In former mobile communication, a radio frequency band is stipulated in response to the mobile communication system (analog or digital) and the number of channels that can communicate at the same time is set depending on the stipulated radio frequency band.
The digital system which allows a larger number of channels to be set efficiently even on the same radio frequency band have become mainstream as recent mobile communication systems, and portable telephones and portable information machines equipped with a mobile communication function, compatible with the digital mobile communication system are widely used.
However, since TDMA (Time Division Multiple Access) is used as the former digital mobile communication system, a situation in which channels cannot be sufficiently assigned and communication becomes impossible occurs frequently in a district where the users concentrate, such as cities, because of a rapid increase in the number of users. Then, attention is focused on CDMA as a digital mobile communication system enabling a large number of channels to be assigned more efficiently than the TDMA.
In the CDMA, the transmitting party multiplies data signals of voice, etc., of a number of senders by different codes and uses one frequency band to transmit the resultant signals, and the receiving party can multiply the resultant signal by the code of the associated party that the receiving party communicates with, thereby decoding and taking out only the communication signal of the associated party.
The CDMA communication standard specifies that a particular test item of xe2x80x9cantinoise environmental testxe2x80x9d be carried out for mobile communication machines (containing communication base stations and portable communication terminals) compatible with the CDMA, and a noise generator used for the xe2x80x9cantinoise environmental testxe2x80x9d is required.
Since the frequency band used in the CDMA is wider than the frequency band used for each channel in the TDMA, a noise generator used for the xe2x80x9cantinoise environmental testxe2x80x9d requires a capability of generating a noise signal covering the CDMA frequency band.
To generate a wide-band noise signal covering the CDMA frequency band, a noise generator of this kind in a related art converts digital data into analog form to generate a noise signal or converts an appropriate analog noise signal into digital form, then modulates the digital signal to generate a desired noise signal, for example.
However, to generate a wide-band noise signal covering the CDMA frequency band, the noise generator in the related art used for the xe2x80x9cantinoise environmental testxe2x80x9d mentioned above converts digital data into analog form to generate a noise signal or converts an appropriate analog noise signal into digital form, then modulates the digital signal to generate a desired noise signal, for example. Thus, the noise source configuration becomes complicated and software for controlling the A/D converter also becomes necessary, increasing costs of the noise generator.
It is therefore an object of the invention to construct a noise generator of only hardware for generating a wide-band noise signal corresponding to an xe2x80x9cantinoise environmental testxe2x80x9d of CDMA for reducing the costs of the noise generator.
According to the invention as in aspect 1, there is provided a noise generator comprising:
a first noise source for generating a predetermined noise signal;
a second noise source for generating a predetermined noise signal;
a first amplification circuit for amplifying the noise signal generated from the first noise source at a predetermined amplification factor;
a second amplification circuit for amplifying the noise signal generated from the second noise source at a predetermined amplification factor;
a first filter circuit for extracting a first noise signal contained in a predetermined frequency band from the noise signal amplified by the first amplification circuit;
a second filter circuit for extracting a second noise signal contained in a predetermined frequency band from the noise signal amplified by the second amplification circuit; and
a modulation circuit for modulating the first noise signal extracted through the first filter circuit and the second noise signal extracted through the second filter circuit and outputting a noise signal of a desired frequency band.
According to the noise generator of the invention as in aspect 1, the first amplification circuit amplifies the noise signal generated from the first noise source at a predetermined amplification factor, the second amplification circuit amplifies the noise signal generated from the second noise source at a predetermined amplification factor, the first filter circuit extracts a first noise signal contained in a predetermined frequency band from the noise signal amplified by the first amplification circuit, the second filter circuit extracts a second noise signal contained in a predetermined frequency band from the noise signal amplified by the second amplification circuit, and the modulation circuit modulates the first noise signal extracted through the first filter circuit and the second noise signal extracted through the second filter circuit and outputs a noise signal of a desired frequency band.
Therefore, the noise generator can be constructed of only hardware and the costs of the noise generator can be decreased.
In this case, as in the invention as in aspect 2, preferably, in the noise generator as in aspect 1, the modulation circuit is a quadrature modulation circuit for quadrature-modulating the first and second noise signals based on a local signal of a desired angular frequency supplied from an external local signal source and outputting a noise signal of a desired frequency band.
According to the noise generator of the invention as in aspect 2, the modulation circuit is a quadrature modulation circuit for quadrature-modulating the first and second noise signals based on a local signal of a desired angular frequency supplied from an external local signal source and outputting a noise signal of a desired frequency band, whereby a noise generator for generating a wide-band noise signal on a desired frequency band determined by the frequency of the local signal and the configuration of the first and second filter circuits can be constructed easily; for example, a noise generator for generating a noise signal that can be used with an xe2x80x9cantinoise environmental testxe2x80x9d of CMDA can be provided.
As in the invention as in aspect 3, preferably, in the noise generator as in aspect 1 or 2, amplification factor setting of both or either of the first and second amplification circuits can be made variable and a capacitor for cutting a DC component of the amplified noise signal is connected to the output stage in series. The capacitor for cutting the DC component can eliminate the need for considering the DC components in the circuits at the stages following the amplification circuit, caused by DC offset voltage generated in the amplification circuit particularly when the amplification factor is large.
According to the noise generator of the invention as in aspect 3, amplification factor setting of both or either of the first and second amplification circuits can be made variable, thus a capacitor for cutting a DC component of the amplified noise signal is connected to the output stage in series, whereby flattening the noise signal level in the proximity of the center angular frequency xcfx89 and the phase component for the frequency characteristic in the proximity of the center angular frequency of the noise signal output from the quadrature modulation circuit can be easily adjusted by varying setting of the amplification factor of the first or second amplifier.
As in the invention as in aspect 4, preferably, in the noise generator as in any of aspects 1 to 3, each of the first and second filter circuits is made of a low-pass filter circuit.
According to the noise generator of the invention as in aspect 4, each of the first and second filter circuits is made of a low-pass filter circuit, whereby a noise signal of a desired frequency band can be extracted easily.
As in the invention as in aspect 5, preferably, in the noise generator as in any of aspects 1 to 4, each of the first and second noise sources is made of a Zener diode circuit.
According to the noise generator of the invention as in aspect 5, each of the first and second noise sources is made of a Zener diode circuit, whereby the costs of the noise generator can be decreased.